The present invention relates generally to control logic for fuel controls on an auxiliary power unit (APU) and, in particular, to a control logic that allows the electronic control to command fuel flows below the programmed lean blowout limit when certain conditions are true.
In addition to their traditional propulsion functions, gas turbine engines are often used as APUs to supply mechanical, electrical and/or pneumatic power to a wide variety of aircraft systems. For example, the APU can be used to start the main engines, supply compressed air to the aircraft's environmental control system, or provide electrical power. Historically, APUs have only been operated when the aircraft was on the ground.
Recent developments in aircraft design have witnessed the advent of twin engine aircraft capable of long distance, transoceanic flights. A disadvantage to the twin engine design is that when a main engine experiences an inflight shutdown, the enormous burden of supplying the aircraft with power falls on the sole, remaining engine. Early on in the development of these aircraft, it was recognized that they would need an additional source of power while inflight. To meet this need, it was proposed to start and operate the APU inflight.
A gas turbine APU includes in flow series arrangement a compressor, a combustor, a turbine and a shaft coupling the turbine to the compressor. During a normal, sea level start, a starter motor applies a starting torque to the APU's shaft. As the shaft starts to rotate, air is inducted into the compressor, compressed and then discharged in the combustor. Concurrently, the APU's fuel control system feeds fuel into the combustor in accordance with a predetermined fuel schedule to precisely maintain the proper fuel to air ration in the combustor. At a rotational speed of about 10 to 20 percent of the APU's operating speed, the condition in the combustor becomes such that the fuel/air mixture can be ignited. This condition is generally referred to as light-off. Should the fuel to air ratio be either too rich or too lean, light-off will not occur and the APU will experience a hung start. After light-off, the start motor torque is augmented by torque from the APU's turbine. At about 50 percent of operating speed, the start motor is shut off and the APU becomes self-sustaining and accelerates itself to operating speed.
To start an APU at high altitude (e.g., 40,000 feet) after the APU has become cold soaked by continuous exposure to cold ambient temperatures (e.g., −70F) is a much more difficult task for the APU's fuel control system. The cold temperature increases the APU's drag necessitating greater starting torque. Further, the cold fuel poorly atomized. Poor atomization combined with low air density makes it both difficult to precisely obtain the necessary fuel to air ratio to accomplish light-off, and to provide a sufficient fuel flow rate to the combustor to prevent blowout while not providing too high a fuel flow rate which may result in excessive turbine inlet temperatures.
Conventional fuel control logic has two significant problems. First, as mentioned above, starting APUs at high altitudes often result in over-temperature shutdowns due to tolerances in the fuel control during low fuel flows. These tolerances may include the difference between the actual fuel flow vs the milliamp (ma) command fuel flow based upon the conventional fuel control logic. When an over-temperature condition is detected, the fuel supply is cut back. However, conventional fuel control logic limits the fuel flow temperature cutback to a minimum command to prevent blowout on a nominal fuel control. In other words, there is a pre-programmed lean limit to the minimum fuel flow that may occur. This pre-programmed lean limit is determined at a level to avoid blowout of the engine.
Low fuel flows may be difficult to accurately measure and, therefore, conventional fuel control logic may require the use of a fuel flow feedback mechanism to calibrate the commanded fuel flows. However, degradation in these fuel feedback mechanisms as well as other engine tolerances often has an effect on the true lean stability limit, which may be lower than the pre-programmed lean limit. Optionally, the fuel flow at low flows may be measured to tighter standards. However, both of these approaches may result in a significant cost impact to the system design.
A second problem with conventional fuel control logic occurs during on-speed operation of APUs (constant speed) at high altitudes. Here, tolerances in the fuel control during low fuel flows may cause the speed of the APU to react slowly to unloading of electrical loads. The fuel controls limit the fuel flow cutback to a minimum command to prevent blowout on a nominal fuel control. However, engine overspeed may occur because the fuel flow is required to be at or above a minimum, preprogrammed fuel flow. As with solutions to the first problem, low fuel flows may be difficult to accurately measure and, therefore, conventional fuel control logic may require the use of a fuel flow feedback mechanism to calibrate the commanded fuel flows. Optionally, the fuel flow at low flows may be measured to tighter standards. However, both of these approaches may result in a significant cost impact to the system design.
U.S. Pat. Nos. 5,274,996 and 5,303,541, issued to Goff et al., describe using a closed loop system on measured fuel flow to more accurately control fuel flow and improve starting reliability. The commanded fuel flow may be trimmed until it matches measured fuel flow. The APUs of Goff, however, may experience fuel flow meter failures and fuel tolerance problems, causing failures of the APU to start at high altitudes.
U.S. Pat. No. 4,128,995, issued to Toot, discloses a method and apparatus for stabilizing an augmenter system. More specifically, the patent discloses stabilizing a turbofan at high speed, high altitude flight conditions by reducing the maximum augmenter fuel/air ration in response to certain pressure and temperature conditions. The Toot patent specifically addresses a rich stability problem within the combustor. The reference does not discuss the issues of lean stability and minimum fuel flow tolerances.
As can be seen, there is a need for an improved fuel control logic that will allow the electronic control to command fuel flows below the pre-programmed lean blowout limit when certain conditions are true, thus avoiding overspeed and over-temperature conditions.